April 23: Your child may be swooning on the school playground and the display board at the city centre may read 44.2 degrees Celsius.

But no point getting hot under the collar. The official maximum temperature in Calcutta is still 38.9 degrees Celsius — 2.1 degrees less than what could be termed a heat wave.

The Alipore Met office pegged the maximum temperature on Wednesday at 38.9 degrees based on a reading around 2.30pm.

However, around 2pm, the reading on a multi-thermometer on the third-floor roof of The Telegraphs office in the heart of Calcutta was 42.7 degrees Celsius. By 3pm, the multi-thermometer that can measure up to 150 degrees Celsius, was reading 43 degrees Celsius.

At 2.42pm, a digital display board maintained by the state governments pollution control board read 44.2 degrees Celsius — well above the heat-wave milestone of 41 degrees Celsius.

So, whose maximum temperature can you warm up to, based on which schools can change timings or declare holidays as some other states in the region have done?

Not The Telegraphs, not the pollution control boards and not even the reading of your tormented body that is screaming it is hotter than what the official figures say.

Because none of the above has a wooden box called Stevensons screen. If air temperature has to be measured according to the norms laid down by the World Meteorological Organisation (WMO) in Geneva and apparently followed world-wide, this box is needed.

According to the WMO norms, the thermometer is kept inside a well-ventilated wooden box known as Stevensons screen. The door to the box opens to the north in the Northern Hemisphere. The box is kept in an open field at a height of 1.25 metres so as not to be affected by the earths low-level radiation, said G.C. Debnath, the director of weather section at the Regional Meteorological Centre in Alipore.

The box is named after its creator Thomas Stevenson, a lighthouse design pioneer and the father of novelist Robert Louis Stevenson. If a thermometer is held in the open (as The Telegraph did), the air temperature is coupled with radiant energy. That will be erroneous reading of air temperature, he added.

Officials said Stevensons screen is painted white to reflect incoming radiation from the sun. The box opens to the north because the objective is to measure temperature from a neutral direction. As the sun moves from the east to the west, the reading of a thermometer facing north is unlikely to be influenced by the suns motion.

The place where the Alipore Met office keeps the screen is open — the maximum temperature reading is done around 2.30pm, the hottest part of the day — but it is surrounded by the leafy environment familiar around the colonial structures of Calcutta.

Other meteorologists said urban centres with a high density of concrete buildings and other structures were likely to be hotter than the outskirts of cities or zones with low density of buildings.

Scientists sometimes call this the urban heat island effect. When the temperature is 39 to 40 degrees Celsius at Calcuttas Dalhousie Square, it might be 35 or 36 degrees Celsius a little beyond Howrah, according to a scientist with the India Meteorological Department (IMD) in Calcutta.

A city centre could be 2 to 4 degrees hotter than the outskirts, the IMD scientist said.

This suggests that the ground reality can be captured better if the Alipore Met office takes readings from spots closer to the centre of the city or from multiple points.

The Met office has a temperature reading centre in Dum Dum, too. But Dum Dums maximum temperature — on Wednesday, it was almost a degree higher than that of Calcutta — is not taken into account for the city as it falls in North 24-Parganas.

Temperature also depends on the amount of heat reflected by the local environment. A rooftop measurement is likely to lead to an elevated reading because of the heat reflected by the surrounding concrete. But there should be no significant difference between temperature on the ground and at altitudes of 20 metres to 40 metres.

Debnath said the current hot spell could continue for the next 24 hours. The westerly wind is still blowing at the upper level, bringing in hot and dry air from the western part of the country.